Filament Dehumidifier

Random or Off Topic
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Inspired in part by
New MK4 is out! - #612 by bitingmidge and
Building a filament dry cupboard?
And this video of Adam Savage’s cabinet https://www.youtube.com/watch?v=taeS72yO__k
And CNC kitchen experiments https://www.youtube.com/watch?v=6tHInlFfMcM

Here are some thoughts:

  1. Desiccant doesn’t really “dry” as much as it keeps the humidity stable
  2. I don’t like Adam Savage’s solution because that dehumidifier seems like the wrong tool for the job and will waste power
  3. A room dehumidifer tries to get a high volume of air to be not too wet
  4. The task requires a small volume of air to be as dry as possible
  5. A small hose could pump a small amount of very dry air into a box
  6. Multiple boxes could be connected in series

Requirements are:

  1. Must produce a small volume of extremely dry air
  2. Must be as low cost as possible, <$50 preferred, <$100 mandatory
  3. Must consume as little power as possible (extension of #2, TCO)

Here is a concept. Suppose you have a TEC and you have two chambers with silica gel, and the TEC establishes a mild temperature differential. (The hot side does not need to be extremely hot, nor does the cold side need to be extremely cold.)

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Now suppose you have a closed cycle air pump that circulates air between the two sides while the temperature is maintained.
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Because of the temperature differential, the cool side will accumulate more water, and the warm side will accumulate less water. Note that the warm side will release water to the cool side even if the warm side is still below the normal regeneration temperature that releases water to ambient air. It should be the same mechanism by which a warm, wet surface in a closed system will cause condensation on a cooler surface, regardless of whether the temperature is near boiling. Only the temperature differential is necessary.

Now after a while, suppose the temperature were reversed and each were vented to the atmosphere. The side with more water (left side), now warm, will reject moisture to the ambient, and the side with less water (right side), now cool, will absorb moisture from the ambient.
image

In this system the moist air and dry air depends on the temperature differential, where a high temperature differential is effectively regenerating the silica gel the old fashioned way by getting it hot. But the point is that even with a mild temperature differential, the system will produce a mild drying effect.

The downside is that the drying effect is rather weak, but the advantage is that nothing gets very hot, and the heating/cooling is energy efficient.

So the next step is to stage these like a bucket brigade.
During the “A” cycle, there are three closed loops and alternating warm/cool chambers:
image

The heated chambers will transfer their moisture to the chamber to the left.

Then, after a while, the cycle changes to the “B” cycle, with two closed loops and two open loops:
image

Each individual stage with a modest temperature difference can only achieve a modest humidity difference, but when they are stacked in series like this, I think each cool chamber of desiccant can pull the humidity of the next warm chamber even lower, so that the dry chamber on the far right can be extremely dry, even with a modest temperature difference.

The TECs are cheap and with some 3D printed ducts and stuff, maybe this whole thing can be built for between $20 and $50.

Thoughts?

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Let’s figure out how to build some prototypes.
I’d love this even for the dryness here in Colorado.

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Edit: Ignore this, I ENTIRELY missed that Jamie was talking about silica gel, I has assumed the two volumes were just air.
Do you have a reference showing that effect? That is counter to my understanding of the topic. I would have said that the relative humidity on the cold side will be higher because of the relative part of the name, but the quantity of water in the air on both sides remains constant. The only way water will ‘accumulate’ on the cold side is if it condenses.

The comment you’ve made about water condensing on a cold surface regardless of whether the temperature is near boiling is more about the dew point temperature of that air. The differential is necessary under most circumstances but it’s less about differential and more about the dew point itself. Taking a fixed volume of air, heating it and then cooling it won’t cause condensation unless the surface doing the cooling is below the dew point of the air. If you take a fixed volume of air with 65% humidity at 20°C then the dew point is 13.2°C. If you heat it to 40°C then the relative humidity will become 20% and the dew point will remain the same. In both cases you would need to provide a surface that is 13.2°C or colder to see condensation.

I really like the idea of trying to come up with a better physics based drying system as the existing applications seem pretty clunky. I think it’s worth establishing a ‘best practice’ baseline, though. For me that has proven to be a slightly modded Creality filament dryer. We’re in a pretty humid climate here (typically 60-80% downstairs, I can pull the data from my temp/humidity sensors and check). Putting a fresh roll of PETG into the dryer and leaving it to equillibrate shows a humidity around 45% in the dryer itself (the PETG ‘drying’ the air). Running the dryer for a couple of hours with the door cracked and then closing the door once everything has cooled resulted in the humidity being ~15% in the dryer. After a month of sitting the humidity is up to 25% despite the humidity in the surrounding room never dropping below 60%. I don’t have any numbers to bring that back to an actual dryness in the filament, but my assumption is that if the PETG were absorbing moisture rapidly then it would max out and the humidity would rise.

I’m also always skeptical of anything TEC related due to the power consumption required to do any level of work with a significant temperature differential. If I get a sec I’ll try work out some numbers around the actual energy consumption of that approach.

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I’d also potentially consider this as two separate topics. One is actually drying the filament itself, the other is maintaining the filament to prevent it rehydrating.

To dry the filament, I’m not convinced you need to actually have anything that’s dehydrating or specifically lowering the absolute humidity. Simply lowering the relative humidity via heating while also lowering the water capacity of the filament by heating it seems to work relatively well. There are obviously efficiency issues with heating something to dry it but if that’s the only issue then at least there’s a clear goal there. Perhaps that’s more a question of finding the right balance of heat vs time. Long times at very low humidity may be more efficient than short times at very high temperatures, at the expense of convenience.

To store the filament, I think that comes down to good enclosure design. That’s something I’ve seen enough of the ‘wrong way’ to know it’s complicated but not enough of the ‘right way’ to know for sure how the hell to do a good job of it. Fundamentally it’s just a case of preventing any air exchange but that can be easier said than done. Any enclosed airspace that heats/cools during the course of a day will naturally pressurize/depressurize leading the the enclosure ‘breathing’. A lot of enclosures are designed such that the seals aren’t good enough to completely prevent air movement with even a small change in pressure and so will breathe very slightly as the temperature cycles naturally. How much of an issue that is depends on how sensitive the filament is to moisture, how long the moisture needs to be kept out and whether there’s anything in the enclosure to buffer it. If the filament is relatively insensitive and will be used within a week, an enclosure probably isn’t needed. If the filament is super sensitive and may be stored for months between uses, that’s a much harder nut to crack and at some point the solution may be to make repeat drying part of the process, rather than trying to hermetically seal a filament roll…

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My mental model is that the silica gel acts like a “reservoir” of relative humidity. If silica gel is in a sealed environment and the air is heated, the relative humidity of the air will initially drop due to increased air temperature, and the silica gel will emit water vapor to balance the RH of the air with the reservoir. When the silica gel and air is cooled, the RH of the air will rise and the silica gel will absorb the moisture from the air to re-balance the RH of the air with the reservoir, even if it never drops below the dew point.

So yes, when the air moves from the warm side to the cool side, the RH of the air will go up (because it’s cooler) and my claim is that the silica gel will reach an equilibrium with a higher absolute quantity of water in the silica gel. The increase in RH will drive the water into the silica gel on the cool side.

As for condensation on cold surfaces, yes in an open system it’s all about dew point, but for a closed system, the warm, wet side will “try” for an equilibrium at 100% RH at the warm temperature, but that humidity level is above the dew point for the cold side, so condensation occurs.

The evaporation from liquid and condensation to liquid is just an analogy though because adsorption into silica gel reaches different quantities at different RH levels, unlike condensation, which does not occur at all until the dew point is reached.

I could be wrong though, which is why I’m posting this.

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Last things first- I have had ASA and TPU bagged and vac sealed with silica gel for almost a year and “think” that it’s OK.

I agree entirely with all you have said, I just can’t find the solution for now.

The best dryer, I have is a food dehydrator - the el-cheapo circular type that has a fan in the bottom, a heater and a vent in the top. By weight, the first hour or two are by far the most efficient, I usually give all filaments about three to four hours, but after the initial drip returns are diminishing.

I also have an Eibos Cyclopes, which is very disappointing - it works on the same principal but air circulation is poor so a stationary spool of filament gets warm on the bottom (sometimes very hot), it’s a pain to change filaments, but it does maintain a very low RH while printing.

Each of my dry boxes has 120 grams of silica gel - I am going to up that to 200 to see if it makes a difference, but with freshly dried filament and moderate conditions - temp in the high twenties with RH in the seventies , I can’t maintain them below 20% for an extended period.

In more extreme proper tropical conditions (Innisfail and Tully in NQ) we have had low wattage light bulbs in our cupboards to keep them a little drier than the outside and prevent mildew. I understand but don’t understand how such a small temp differential works, but it does!

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I just took a look at the TEC performance required assuming it was used purely as a condensating dehydrator.

If we assume we want to get the relative humidity down to 30% at 20 degrees C, that’s a dew point of 1.9°C. Any lower than that and things get awkward due to freezing.

If we’re trying to remove 2% moisture from a 1kg roll of filament, that’s 20g of water. Heat of vaporization is 2.2 kJ/g so that’s 44kJ of energy used purely to condense the water out of the air. Making the assumption that we do this over an hour, that’s 12W. If we say that we’re going to control the cold side of the peltier to 2°C and put enough heat-sink to keep the hot side to 40°C then from checking the datasheet for a decent 40x40 peltier module that looks like around 3.5A at 10V, so 35W, which is actually better than I expected. That’s assuming we have zero leakage of energy from the hot side to cold side and that we’re doing nothing to cool anything other than condensing water out of the air. In reality I suspect that’d be both a longer operating time and higher thermal performance needed so I suspect that’s low by 10x, maybe? So assuming 350Wh to get that 20g of water out. That’s in the range of heating and silica gel, energy wise.

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Speaking of data here is a chart that CNC Kitchen references

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The implication is that RH is what determines adsorption quantity.

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Yeah, that’s about right. It’s a reservoir that has an equillibrium point in terms of absolute capacity that varies wildly with temperature/relative humidity.

That’s my mistake, I didn’t see that you’d mentioned that there was silica gel in each scenario, I thought this was purely an airspace consideration.

I’m missing something here, how are you proposing that this would read 100% on a warm situation unless the system is close to saturation, which is likely way above where we’re dealing with if we want to reach dry conditions at reasonable room temperatures?

So in this case, that’s a lot like how the typical self-regenerating silica gel dryers work. Plug it into the wall and a heating pad heats the silica gel, lowering the relative water holding capability and causing it to lose moisture in an absolute sense. Then unplug it, cool it down and put it back where it was previously. That seems much more reasonable. I think the key thing I missed was the presence of silica gel which adds a non-linear and exploitable element to the whole thing.

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I am wrong! I am somewhere between a pure science believer and a denier on this - such is my confusion! Last time I shared all this angst - I built a beautiful wall of dry boxes which only work if they are constantly maintained, and drying twenty canisters of silica gel every month is a pain!

It’s made worse by the fact that I rarely have problems with filament (even sometimes printing wet PETG is OK - depending on the model and number of retractions of course) and I know there are plenty of other people who apparently have no problems despite living in similar conditions (Hi @Jonathjon :wave: )

I have done enough reading to be a complete peltier sceptic and a “heat pump” believer. No science - just a view formed by indoctrination.

I can buy a cheap compressor dehydrator (like Adam Savages) for around $100US, a low wattage heating element for a few dollars, and a computer fan for free.

It would be trivial in terms of power for me to keep the air in the cupboard at say 35°C, if the cupboard were sealed and a dehumidifier installed Savage style - could I not just use a sensor to turn it on when the RH reached say 30% and off at 20?

Perhaps a few kg of silica would help stabilise this too.

Given that mostly I print with PLA and the vac bag seems to work with more hygroscopic material - this is a really huge overthink, but that’s what I do best.

Besides I could keep my camera gear in it as well and that would make me very happy!

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That’s an interesting test. There are two aspects here that I think are likely important to consider. One is the residual moisture in the filament, the other is potential breathing or moisture movement into the boxes. Both of those will result in the humidity changing over time.

If the silica gel were a perfect absorber that simply sucked up water out of the air any time the humidity exceeded a certain amount you’d see the humidity rise up to that point, stay constant until the gel was saturated and then continue to increase.

In a similar vein, the filament itself (and potentially other things like the cardboard of the reels etc.) will have a water content vs relative humidity curve that may have a significantly longer time constant. So under the assumption that the silica gel has a much shorter time constant than the filament, the filament will slowly release moisture until it reaches equillibrium while the silica gel will quickly absorb moisture to reach equillibrium.

To run some numbers (to check my own assumptions, as much as anything):
If you have some silica gel at 5% moisture content then that’s 10% humidity from the chart above. If you have 1kg of filament at 2% more moisture content at room conditions than long term at 10% RH then that’s 20g that needs to be handled to get to that 10% long term. If you had 100g of silica gel in there, that 20g of moisture is 20% of the gel’s weight so would be enough to raise the relative humidity (ignoring air etc.) to 50% relative humidity, so obviously the filament would never actually dry under those conditions. On the other hand if you had 1kg of silica gel in there, that 20g of moisture would be 2% of the gels weight so would raise the humidity by only a few %.

On the other hand, if you pre-dry the filament to the point where it’s no longer losing weight and put it in the dry box then it would be fair to assume that there isn’t 20g of water lurking in the filament anymore, maybe it’s more like 2g at 10% RH. In that case the 100g of gel would pick up 2% by weight so would barely change the RH. If the RH steadily increases beyond that, that means either the box is leaking or the filament wasn’t dried enough. Either way, more gel solves both issues, providing your timeframe is ‘reasonable’.

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I wonder if there’s a way of making one of those automated and on a bigger scale? I am missing how in @jamiek Jamie’s model the silica gets regenerated.

I’m going to keep my eye out for a suitable cupboard in which to experiment - not sure what form those experiments will take - but I am going to move forward this time!

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I live in a relatively high humidity climate. Typically stays between 50-65% RH indoors.

I’ve been having these in these Rubbermaid DIY dry boxes for roughly 3 months now and they haven’t gone above 10%

I print directly from them so there is not much opening of the top. Only when I change filament if the end falls in and I can’t grab it when I unscrew the cap

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Not sure what the difference is, but these boxes seem to seal really well

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Sorry, I am mixing scenarios. The air near the warm silica gel would not reach 100% saturation.

The analogy was with a closed system with no silica gel (maybe the source of the confusion), where evaporation/condensation can occur similar to a heat pipe but slower. The system without the silica gel and with liquid evaporation/condensation was to make the point that water can move without reaching extreme temperatures, and even a small temperature differential makes water move from the warm side to the cool side.

Yes, it is similar, especially the first diagram. The difference being that regenerating the silica gel to ambient might not get as dry as regenerating to a cool silica gel pack that drops humidity below ambient. The other difference is that TEC could maybe be more than 100% efficient compared to a resistive heater, especially when the cold side is also useful. But this efficiency is more difficult when the temperature difference is high.

The multiple stages might not be an improvement. The idea was that with a lower temperature differential, the COP of the TECs would be much higher, but that might be more than offset by the fact that you have to pay to evaporate each gram of water multiple times as it’s handed down the bucket brigade.

Low temperature might have other benefits like less heat losses, but that doesn’t matter if the net efficiency isn’t better. The specifics would determine whether the evaporation or the other losses were the dominant energy sink.

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I think it depends on scale. Heat pumps are awesome at larger scales but they suck for small applications. Using one to dry a single roll of filament, for instance. In this case I think the TEC could be useful, but it would need to provide some kind of benefit over exploiting the curve of silica gel vs free air to be worth the hassle/inefficiency.

I think I’ve commented before that I think dehumidifiers are an underrated investment for most people. I have one in my moderately sealed garage that runs any time the humidity rises above 60% and it costs me maybe $200 a year to run? That’s a solid trade-off for protection of the tens of thousands of dollars worth of tools, toys etc. that live in there. That’s also considering that maybe 60 days of the year a wet car gets parked in there alongside everything. Without that detail, it’d be remarkably less to run.

I think there would be considering that it’s basically what I’m doing with my filament dryer but manually. Silica gel in the dryer, run it hot with the lid cracked for a few hours which dries the silica gel and the filament. Let it cool then close the lid.

As I said above, I think it’s worth separating into 2 stages and then seeing if that needs to be combined. First is drying, second is storage. Maybe nothing needs to be automated if it goes in dry and has the right combination of desired storage time, humidity buffer and sealing. If not then something that can do the drying phase automatically would help.

There was a dry-box setup I saw somewhere, I think 3D Printing Nerd, that was a sealed plastic food storage container looking thing that had bungs that could be removed so it could be placed on a dedicated heater/dryer. It looked really good to me and looked like a combo of all these things.

Ah, it was the PolyDryer from Polymaker:
https://polymaker.com/introducing-polydryer/

I haven’t looked into it too much further but that seems like a decent thing to look at in comparison.

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The difference between these two scenarios is very interesting. There are really only a couple of things it can be which can potentially be tracked down. One is that the filament going in is drier, one is that the expectations are different (different acceptable RH vs time, different sensitivity to wet filament defects, different filaments used), one is that there is more air exchange.

One thing that I keep coming back to his how sealed the boxes are and how much temperature fluctuation there is. I have my PETG in a basically unsealed Creality Space Pi downstairs in ~70% RH and it hasn’t increased much in humidity over the ~2 months since I last dried it (15% to 25% at ~20°C, according to the dryer). I even thought I had silica gel in there but didn’t because I’d pulled it out with the last partial roll of filament and put it in the ziplock back with it. Obviously that’s still a pretty big change, but the thing is just really a plastic lid and isn’t sealed at all.

One other thing could be a difference in expectations vs temperature. The Sorbent Systems graph that CNC Kitchen used above is for a single constant temperature. If you’re seeing the temperature fluctuate then you will also see the RH fluctuate alongside as the capacity of the silica gel changes?

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Here is what I am going to do!

I suspect there is leakage in the boxes - but they aren’t pressure vessels so just why a volume of water in the air is so keen to osmotically transfer itself is not clear to me.

  1. I have just ordered a new food dehydrator . I stupidly perhaps gave my old one to my grandson, thinking that the Eibos would be the answer!

  2. I will set up three boxes and monitor them over a few months on this thread. This will prevent me from getting sidetracked and actually building anything during that time!

  3. I will dry and document as much as I can, continue to use the Eibos while printing, and decide where to go after that.

  4. I will probably buy the dehumidifier too (the compressor one) as we have are a number of other uses for it - and who knows what the next stage will be?

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This is something I’d like to know more about, as to how the RH vs. equilibrium moisture content (EMC) curve changes with temperature. Is it largely consistent over a range of temperatures or does it change dramatically? At high temperature, silica gel might not decrease capacity so much as air increases capacity. Regeneration can still work even if basically the same curve applies at high temperatures, it’s just that modest RH at high temperatures is super high absolute humidity.

I doubt RH vs. EMC stays precisely the same over temperature but maybe for practical purposes the relationship is effectively constant?

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I print from the same thing as @Michael_Melancon does. But I don’t have the fancy caps lol.

And this is what I dry filament with…

Elite Gourmet EFD319BNG Food… Amazon.com: Elite Gourmet EFD319BNG Food Dehydrator, 5 BPA-Free 11.4" Trays Adjustable Temperature Controls, Jerky, Herbs, Fruit, Veggies, Dried Snacks, Black and Grey, 5 Trays: Home & Kitchen

I cut all of the plastic out of the rings and I can dry 2 spools at a time. If I am not quite ready to print with it I have these totes that @Michael_Melancon suggested to me and they work really well. I have the entire bottom covered in about a half inch of decadent

https://www.walmart.com/ip/Sterilite-54-Quart-Clear-Gasket-Box-with-Blue-Latches-Gasket/39922428

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My brain understands the RH goes down as temperature goes up, but I can’t get my head around how that dries stuff - the volume of water hasn’t changed, just the volume of the air - some times I just have to trust what the smart guys say.

I guess the US solution to our climate is turn on an air conditioner, the Aus solution is to whine about it! :smiley: We choose to live here, actually like it mostly, but I’d like to solve this little problem too! :wink:

Here’s tonight’s forecast when the temperature drops to low 20’s -

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